Asia Pacific - The decimation of Borneo’s forests over the past half century is well known. Every tree lost releases carbon into the atmosphere, hastening climate change.

But over the same time period, a new study has found, the forests that remain – the untouched, old-growth forests of the interior – are getting denser.

For millions of years, exuberant tropical vegetation has sprouted, bloomed and died here. The trees absorb and store carbon as they grow, and release it back into the atmosphere when they die and rot away.

Traditionally it was thought that primary forests like these must have reached a kind of carbon equilibrium – that once a forest is mature, new trees would replace dying ones at a steady rate, and the total amount of living material in a given area would remain roughly the same over time.

But that’s not what is happening in Borneo, or indeed in the world’s other tropical forests. An international team of scientists led by Lan Qie, then at the University of Leeds, analyzed long-term measurements of 71-hectare plots across the huge island, and discovered that over the past half century, Borneo’s intact forest increased in biomass by an average of 430 kilograms per hectare per year.

That means it is acting as a carbon sink, says Qie. And Borneo is not alone. Previous studies have shown the undisturbed tropical Amazon and African forests are also absorbing more carbon than they are releasing.

“We now know this is a global trend,” Qie says. “En masse, across the tropics, these intact forests are out of equilibrium.”

That means that primary forests are probably mopping up some of our excess carbon emissions, and helping to mitigate the effects of climate change.

Why could this be? It still needs to be proven experimentally, she says, but the most likely explanation is that there’s a global driver simulating higher levels of growth, possibly linked to the increasing carbon dioxide levels in the atmosphere.

But there’s only so long old forests can do that. “You can imagine that a patch of forest on a fixed land surface simply can’t be getting larger indefinitely,” says Qie.

“We expect that the faster growth we’re seeing now will maintain the carbon sink in the short term, until tree death catches up, and the forest biomass reaches a new, higher equilibrium.”

An indigenous Iban man out fishing in West Kalimantan, Borneo. Many remote parts of the island can only be reached by boat. CIFOR Photo/Lucy McHugh

LOST HERITAGE

To measure the changing biomass over time, Qie’s team used forestry plots first established in Borneo in the 1950s, 60s and 70s. As early as 1958, pioneer ecologists were marking out areas of forest – usually a hectare in size – and returning every few years to record the size, number and species of the trees found there.

Qie unearthed and collated years of data, and in 2013 and 2014 her team set off into remote parts of Malaysian and Indonesian Borneo to track down and re-measure the plots.

Sometimes that was easier said than done, Qie says.

In 2013 she tried to re-locate a plot dating back to 1979, deep in the Gunung Mulu National Park in Sarawak. It had been consistently monitored during the 80s and 90s but not visited since 2000.

It’s like resuscitating some nearly-lost research heritage

Lan Qie, scientist

That was before the invention of GPS, so the only reference Qie had for the plot was a hand-drawn sketch map.

“I could locate it roughly on Google Earth, just by comparing the ridge lines and the river, and the nearby village,” she says.

Then she consulted a “70-year-old Iban uncle” who had assisted the research teams as a young man in the 1970s.

“He was too frail to travel into the national park, so he could only give me instructions: ‘You walk for about 20 minutes, then on your right, you bash through the forest and the plot is a few hundred meters through there.’”

The team spent a whole day searching for the plot. “We had run out of water and were getting dehydrated – this is dense, low-lying, stuffy forest with high humidity and low oxygen levels – and we nearly gave up.”

Finally, one of Qie’s assistants spotted a tiny aluminium plot marker sticking out of a tree about ten metres away. “He was my hero! It was amazing that he spotted it in the dim light beneath the canopy.”

Now, that plot and all the others are GPS-located, and their data stored online for other researchers to use.

“It’s like resuscitating some nearly-lost research heritage,” Qie says.

While overall the study found Borneo’s forests have been acting as a carbon sink, the authors also identified two ways the sink may reverse.

Extreme droughts that kill large numbers of trees can temporarily turn a sink into a carbon source. The study found that during the 1997-1998 drought (influenced by the El Niño climate pattern), Borneo’s forests briefly became a net emitter of carbon. Within a decade they had recovered, and were again storing as much carbon as pre-drought.

The other challenge to Borneo’s sink is continual and ongoing. The study found that stretches of forest close to an edge – adjacent to human activities – are more likely to emit than store carbon.

Qie says this is because trees on the edge of the forest are significantly more likely to die – through logging or other causes – and when they do, they tend to be replaced by other species that have lower wood density and store less carbon.

The edges are multiplying. Over the past four decades, Borneo’s previously unbroken expanse of forests has been fragmented. In 1973, 76 percent of the island was forested. By 2016, that had fallen to 50 percent, after nearly 20 million hectares of old-growth forest was destroyed by fire and agricultural expansion.

Fragmentation and climate effects can compromise the ability of these forests to act as a carbon sink – and in fact speed up the process

Terry Sunderland, scientist

Qie and colleagues’ research underscores the need to halt that fragmentation, says CIFOR’s David Gaveau, who has developed an interactive map that shows the extent and location of deforestation in Borneo and what is driving it.

An updated version of the map launched last November allows users to explore additional data on the location and ownership of palm-oil mills, which can help form a better picture of the impact of industry on forests.

Former CIFOR Principal Scientist Terry Sunderland (now at the University of British Columbia) who hosted Qie in Bogor, Indonesia, during her fieldwork and is a co-author of the resulting study, says her work has set “an incredibly important baseline for future studies.”

“The beauty of this study is that there is very little research in Asia on forests as a carbon sink, unlike in Latin America and Africa. There are considerable ecological and biogeographic differences between these forest types, yet the impacts of disturbance are remarkably similar,” Sunderland says.

“Fragmentation and climate effects can compromise the ability of these forests to act as a carbon sink – and in fact speed up the process.”

“Borneo’s forests are known to be extremely diverse in terms of species per hectare – we now also know about the carbon value of them as well.”

In Bogor, Lan Qie’s research was supported by the USAID biodiversity earmark. The international forest monitoring network of researchers that contributed to this work was developed with support from a European Research Council grant awarded to Professor Oliver Phillips and Professor Simon Lewis, both at the University of Leeds.

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CIFOR advances human well-being, equity and environmental integrity by conducting innovative research, developing partners’ capacity, and actively engaging in dialogue with all stakeholders to inform policies and practices that affect forests and people. CIFOR is a CGIAR Research Center, and leads the CGIAR Research Program on Forests, Trees and Agroforestry (FTA). Our headquarters are in Bogor, Indonesia, with offices in Nairobi, Kenya, Yaounde, Cameroon, and Lima, Peru.